1. Field of the Invention
The present invention relates generally to computers and, more specifically, to power management.
2. Description of the Related Art
A Universal Serial Bus (USB) allows coupling of peripheral devices to a computer system. USB is a serial cable bus for data exchange between a host computer and a wide range of simultaneously accessible devices. The bus allows peripherals to be attached, configured, used, and detached while the host is in operation. USB based systems may require that a host controller be present in the host system, and that the operating system (OS) of the host system support USB devices.
In general, a USB device attached to the host controller of the computer may prevent the central processing unit (CPU) of the computer from entering a low power state—e.g., the C3 state. The host controller, as a bus mastering peripheral, may keep the Peripheral Component Interconnect (PCI) bus active as long as the host controller is attached to a USB device. The active PCI bus may prevent the CPU from going into a low power state.
Portable computers may not be able to provide full, aggressive power and system management of USB devices and peripherals through a USB only interface. The USB interface may not provide sufficient information between the USB device and a power management entity of the portable computer to implement aggressive power management.